1. Field of the Invention
The present invention relates to a method for detecting a position of an object by detecting a signal from a scale serving as a reference for a linear encoder, a rotary encoder, etc., and more particularly to a method for detecting a position so as to receive a position detecting signal which is not affected by pitch error of a scale.
2. Prior Art
According to known methods for detecting a position, photoelectric elements read an occulting light or a moire pattern for each pitch of an optical scale or a moire scale, each such pitch serving as a reference of a linear encoder, a rotary encoder etc., Alternatively one pitch of an electric or magnetic scale is detected by inducing voltage in order to detect a moving direction of a measuring object, finally detecting a position thereof. In other words, a pitch scale of various scales is detected by an optical sensor, an electrical sensor, a magnetic sensor, etc., and a position is detected based on signals detected by these sensors.
In this way, according to the known methods, since the scale to be used as a reference is read by photoelectric or magnetic means and the read information is detected and operated to output a position signal, it is required to minimize error of the scale itself in order to improve a position detecting accuracy. However, even if the error of scale itself is minimized, in case of a cylindrical drum rotary type scanner, for example, another erroneous factor due to rotation of a drum may be a problem since a signal of an encoder coupled coaxially with the drum is used to obtain a main scanning position signal. In other words, since the drum is a rotational body, a little radial swing cannot be avoided, and some error in a recording position on the drum surface is inevitably caused by this radial swing. Moreover, since the drum is connected with the encoder by coaxial coupling in this kind of scanner, an accurate centering is required.
In order to meet the foregoing erroneous factor of the drum, an attempt has been proposed wherein the scales for position signal are formed on the periphery of a drum to be detected.
In this way, even if error of the scale to be detected is minimized, another error in the mechanical system for driving the measuring object is further involved in. Accordingly, in the present state of art, it is generally understood that when reading information from each pitch on the scale by some means to obtain a position detecting signal based on the information, an error between two adjacent pitches as well as an error accumulated through the process of reading is unavoidable to a certain extent.